Mica is classified as a phyllosilicate; its basic structural feature is a composite sheet in which a layer of octahedrally-coordinated cations is sandwiched between two identical layers of linked (Si,Al)O4 tetrahedra. The general formula of the mica structure can be found in Dana's New Mineralogy, R. V. Gaines et al., eds. (John Wiley & Sons, New York 1997), pages 1444-1446 and the structure can be written as:A0-5R2-3T4O10X2 where:                A=a large monovalent or bivalent ion (e.g. Na+, K+, Rb+, Cs+, Ca2+, Sr2+, Ba2+), or a partial vacancy (partial vacancy denoted by subscript “0”),        R=an octahedrally-coordinated cation (e.g. Li+, Mg2+, Fe2+, Mn2+, Zn2+, Al3+, Fe3+,Mn3+, V3+),        T=a tetrahedrally-coordinated cation (predominantly Si4+, with Al3+ and B3+), and        X=is an anion (predominantly OH− in minerals, but F− in glass-ceramics. X may also be partially O2−.)        
Micas are extremely common in rocks, and numerous classification systems exist for them. In glass-ceramics, micas are typically classified as alkaline (containing alkali ions) and non-alkaline (containing no monovalent ions), and as trisilicic (where T4 in the formula above is (Si3Al)) and tetrasilicic (Si4). These compositional parameters can be varied to produce desired properties in a glass-ceramic.
Machinable mica glass-ceramics based on mica crystal phases were originally disclosed in the art more than thirty years ago [e.g. U.S. Pat. Nos. 3,689,296, 3,732,087, 3,839,055, and 3,756,838], and Table 1 below shows general formulas for trisilicic, tetrasilicic and non-alkali glass-ceramics having mica structures, the alkali-containing glass-ceramics being included in the trisilicic and tetrasilicic categories. These materials have found numerous uses based on their unusual capability of being machinable to high tolerance using conventional high speed metal-working tools. By suitably tailoring their compositions and nucleation and crystallization temperatures, a wide range of microstructures can be obtained, including the “house-of-cards” microstructure of relatively large mica crystals with high two-dimensional aspect ratios, which most enhances the inherent machinability of the materials (see W. Höland and G. Beall, Glass Ceramic Technology (Amer. Ceramic Soc., Westerville, Ohio, 2002), pages 7-9 and 236-241. Additionally, one can refer to U.S. Pat. No. 2,920,971 (Stookey), the basic patent in the field of glass-ceramics, which provides an extensive study of the practical aspects and theoretical considerations that must be understood in the manufacture of such articles as well as a discussion of the crystallization.
The machinable mica glass-ceramics typically have an inherent white color. For certain applications, however, especially for consumer-oriented products, a colored machinable glass-ceramic is desired. Of particular interest would be a black or deep charcoal grey color. The addition of colorants to opaque glass-ceramics has been disclosed in the art. Colored canasite glass-ceramics were disclosed in U.S. Pat. No. 5,070,044, but these are not machinable materials. D. G. Grossman in U.S. Pat. No. 3,732,087 discloses the addition of glass colorants, for example, transition metal oxides, rare earth oxides, and colloidal metals such as Au, to tetrasilicic mica glass-ceramics, typically in levels less than 2%. In a preferred embodiment, the colorant was added to the molten glass just prior to forming, leading to a marble-like effect in the final glass-ceramic. While the patent indicates that less than 5% colorant is preferred, claim 2 of the patent covers a total of 0-20% of glass colorants.
Durable glass-ceramic articles for use as the outer shell or body of consumer electronics, for example, cellphones, music players, notebook computers, game controllers, computer “mice”, electronic book readers and other devices, are desirable and possess certain advantages such as weight and/or resistance to impact damage (e.g., denting), over the present materials such as plastic and metal. It is particularly desirable to have colored durable glass-ceramic articles for such uses.